The present invention relates to rectangular parallelopipedic lepidocrocite particles and a process for producing the same, and more particularly, to lepidocrocite particles which have a rectangular parallelopipedic shape and which are individual, and a process for producing such lepidocrocite particles.
The rectangular parallelopipedic lepidocrocite particles according to the present invention are firstly used as a coloring pigment for paints, resin moldings, printing ink, road asphalt, cosmetics, etc., and secondly used as a non-magnetic material for non-magnetic undercoat layers, more particularly, as a non-magnetic material for non-magnetic undercoat layers which are formed on non-magnetic substrates which constitute a substrate for magnetic recording media, and which have an excellent surface smoothness and a high strength.
Firstly, a pigment as a use of lepidocrocite particles will first be explained.
Recently, orange pigments dispersed into vehicles or resins have been widely used as coloring pigments for paints, resin moldings, printing ink, road asphalt, cosmetics, etc. An orange color is a color representing warning in traffic rules, so that orange pigments have come into wide use as coloring pigments for road asphalt and traffic paints. The orange color here is referred to a color having a hue in the `L* a* b* color system` in which the a* value is in the range of 15 to 50 and the b* value is in the range of 20 to 55.
As orange pigments, organic pigments such as Sudan I, permanent orange and lithol fast orange, and inorganic pigments such as orange chrome (PbCrO.sub.4.PbO) and chrome vermilion (PbCrO.sub.4.PbMoO.sub.4.PbSO.sub.4) has been put to practical use and come to wide use.
However, the organic pigments are generally expensive. On the other hand, the inorganic pigments are toxic because of heavy metals contained therein, such as lead and chrome. Therefore, organic pigments which are inexpensive and nontoxic are strongly demanded.
Iron oxide particles, ferric oxide hydroxide particles, etc. have an excellent environment stability such as good stability to oxidation in the air and nontoxicity, and they also have various hues. Consequently, the ferric oxide hydroxide particles have conventionally been widely used as a coloring pigment for paints, resin moldings, printing ink, road asphalt, cosmetics, etc.
There are various kinds of inexpensive and nontoxic iron oxide particles. Among them, those which are used as orange pigments are a mixture of yellow goethite (.alpha.-FeOOH) particles and red hematite (.alpha.-Fe.sub.2 O.sub.3), and lepidocrocite (.gamma.-FeOOH) particles.
From the point of view of enhancement of working capacity and improvement of physical properties of a coating film, the improvement of various properties of the particles are strongly demanded.
To state this concretely, the enhancement of the tinting strength and the hiding power of a pigment as well as a good dispersibility of the pigment into a vehicle and a resin is strongly demanded.
Secondly, a non-magnetic material for non-magnetic undercoat layers of magnetic recording media as a use of lepidocrocite particles will now be explained.
With a development of miniaturized and lightweight video or audio magnetic recording and reproducing apparatuses for long-time recording, magnetic recording media such as a magnetic tape and magnetic disk have been strongly required to have a higher performance, namely, a higher recording density.
Especially, video tapes have been increasingly required to have a higher recording density, and the frequencies of carrier signals recorded in recent video tapes are higher than those recorded in conventional video tapes. In other words, the signals in the short wave-length region have come to be used, and as a result, the magnetization depth from the surface of a magnetic tape has come to be remarkably small.
In order to enhance the recording density, it is necessary to maintain high output characteristics and to reduce noise, especially to enhance the S/N ratio even with respect to signals having a short wavelength. For this purpose, in a magnetic recording medium composed of a substrate and a magnetic recording layer formed on the substrate, it have been conducted to reduce the thickness of the magnetic recording layer, as described in the following literature.
For example, Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder, published by Sogo Gijutsu Center Co., Ltd. (1982) states on page 312, ". . . the conditions for high-density recording in a coated-type tape are that the noise level is low with respect to signals having a short wavelength while the high output characteristics are maintained. To satisfy these conditions, it is necessary that the tape has large coercive force Hc and residual magnetization Br, . . . and the coating film has a smaller thickness".
Development of a thinner film for a magnetic recording layer has caused some problems.
It is necessary to make a magnetic recording layer smooth and to eliminate the non-uniformity of thickness. As well known, in order to obtain a smooth magnetic recording layer having a uniform thickness, the surface of the substrate must also be smooth. This fact is described on pages 180 and 181 of Materials for Synthetic Technology-Causes of Friction and Abrasion of Magnetic Tape and Head Running System and Measures for Solving the Problem (hereinunder referred to as "Materials for Synthetic Technology") (1987), published by the publishing department of Technology Information Center, ". . . the surface roughness of a hardened magnetic layer depends on the surface roughness of the substrate (back surface roughness) so largely as to be approximately proportional, . . . since the magnetic layer is formed on the substrate, the more smooth the surface of the substrate is, the more uniform and larger head output is obtained and the more the S/N ratio is improved".
Further, there has been caused a problem in the strength of a substrate film or the like with a tendency to reduction in the thickness of a non-magnetic substrate such as a base film which has conventionally been used as a substrate for a magnetic recording layer. This fact is described, for example, on page 77 of the above-described Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder, ". . . Higher recording density is a large problem assigned to the present magnetic tape. This is important in order to shorten the length of the tape so as to miniaturize a cassette and to enable long-time recording. For this purpose, it is necessary to reduce the thickness of a substrate . . . With the tendency to reduction in the film thickness, the stiffness of the tape also reduces to such an extent as to make smooth travel in a recorder difficult. Therefore, improvement of the stiffness of a video tape both in the machine direction and in the transverse direction is now strongly demanded".
As described above, with a tendency to reduction in the thickness of a magnetic recording layer, it is strongly required that the substrate for forming the magnetic recording layer has as smooth a surface as possible, and a strength high enough to compensate for a reduction in the strength of a non-magnetic substrate such as a base film which is sacrificed by a tendency to reduction in the thickness.
Various efforts have been made to improve a substrate for a magnetic recording layer. For example, a substrate composed of a non-magnetic substrate such as a base film and at least one undercoat layer (hereinunder referred to as "non-magnetic undercoat layer") obtained by dispersing non-magnetic particles into a binder, which is formed on the non-magnetic substrate, has already been put to practical use (Japanese Patent Application Laid-Open (KOKAI) Nos. 63-187418 (1988) and 4-167225 (1992)).
Japanese Patent Application Laid-Open (KOKAI) No. 63-187418 (1988) proposes a magnetic recording medium comprising a non-magnetic substrate, at least one undercoat layer produced by dispersing non-magnetic particles in a binder, and a magnetic layer produced by dispersing ferromagnetic particles in a binder, wherein the ferromagnetic particles are ferromagnetic iron oxide particles, ferromagnetic cobalt-modified iron oxide particles or ferromagnetic alloy particles, the average major axial diameter of the ferromagnetic particles measured through a transmission electron microscope is less than 0.30 .mu.m and the crystalline size thereof by X-ray diffraction is less than 300 .ANG..
Japanese Patent Application Laid-Open (KOKAI) No. 4-167225 (1992) proposes a magnetic recording medium produced by forming a magnetic layer on the surface of a non-magnetic substrate through an undercoat layer which contains acicular particles having an aspect ratio of more than 3.0 in a resin binder hardened when irradiated with an electromagnetic wave such as radioactive rays and ultraviolet rays.
Although a substrate having as smooth a surface as possible and a high strength is now in the strongest demand with a tendency to reduction in thickness of not only a magnetic recording layer but also a non-magnetic substrate, no substrate ever obtained has such properties.
The non-magnetic undercoat layer described in Japanese Patent Application Laid-Open (KOKAI) No. 63-187418 (1988) is produced by dispersing non-magnetic particles such as .alpha.-Fe.sub.2 O.sub.3 (.alpha.-iron oxide) and .alpha.-Al.sub.2 O.sub.3 (.alpha.-alumina) into a binder. Since the shapes of the non-magnetic particles are acicular or granular, it is impossible to say that the acicular or granular non-magnetic particles can adequately improve the surface smoothness and the strength of the non-magnetic substrate.
The non-magnetic undercoat layer described in Japanese Patent Application Laid-Open (KOKAI) No. 4-167225 (1992) is produced by dispersing acicular .alpha.-FeOOH particles in a binder, but since they have also an acicular shape, it is impossible to say that the acicular .alpha.-FeOOH particles can adequately improve the surface smoothness and the strength of the non-magnetic substrate.
Examples of the conventional processes for producing lepidocrocite particles are (1) a process for producing lepidocrocite particles by passing an oxygen-containing gas, at a temperature of not more than 15.degree. C., into a suspension containing a ferrous hydroxide and having a pH of 7.0 to 9.0 which is obtained by reacting a ferrous salt solution and an alkali solution (Japanese Patent Publication No. 33-6734 (1958)), (2) a process for producing lepidocrocite particles by passing an oxygen-containing gas, at a temperature of 5 to 15.degree. C., into a suspension containing a ferrous hydroxide and having a pH of 5.5 to 7.0 which is obtained by reacting an aqueous ferrous sulfate and an aqueous alkali solution (Japanese Patent Application Laid-Open (KOKAI) No. 55-3323 (1980)), (3) a process for producing lepidocrocite particles by passing an oxygen-containing gas into a suspension containing a ferrous hydroxide and having a pH of less than 5.5 which is obtained by reacting an aqueous ferrous sulfate and an aqueous sodium hydroxide, in the presence of disodium hydrogenphosphate at a temperature of about 45.degree. C. (Japanese Patent Application Laid-Open (KOKAI) No. 62-108738 (1987)), and (4) a process for producing lepidocrocite particles by passing an oxygen-containing gas into a suspension containing a ferrous hydroxide and having a pH of about 10 which is obtained by reacting an aqueous ferrous sulfate and an aqueous alkali hydroxide, in the presence of a water-soluble phosphorus compound or arsenic compound to produce lepidocrocite seed crystals, and growing the lepidocrocite seed crystals in a temperature range of 55 to 100.degree. C. (Japanese Patent Publication (KOKOKU) 43-2214 (1968)).
Although a process for industrially and economically producing orange pigments which have excellent dispersibility, tinting strength and hiding power, and which are inexpensive and nontoxic is now in the strongest demand, the orange pigments composed of the above-described iron oxide particles cannot be said to adequately meet such demands.
Mixed particles of yellow goethite (.alpha.-FeOOH) particles and red hematite (.gamma.-Fe.sub.2 O.sub.3) particles are disadvantageous in that since different kinds of particles are mixed, the dispersibility of the mixed particles in a vehicle or a resin at the time of producing a coating material or the like is not sufficient, and in that after dispersion, segregation in the coating material or the like is likely to be caused.
When lepidocrocite particles are produced, acicular particles are likely to agglomerate due to a production reaction. The dispersibility of such lepidocrocite particles into a vehicle or a resin is therefore not sufficient. In addition, the tinting strength and the hiding power are not sufficient due to the particle form.
This fact is explained in the following.
When lepidocrocite particles are produced, an aqueous ferrous chloride solution or an aqueous ferrous sulfate solution is generally used as an iron material.
If an aqueous ferrous chloride solution is used as an iron material, a reaction vessel or the like is corroded, which is industrially disadvantageous.
On the other hand, use of an aqueous ferrous sulfate solution as an iron material is free from the corrosion of a reaction vessel, but the particles produced by any of the above-described methods (1) to (3) are acicular particles. In addition, particles other than lepidocrocite particles are likely to mix with the lepidocrocite particles due to the production reaction.
When the pH of the solution in the reaction process is in the acidic range, acicular goethite particles mix with lepidocrocite particles, and when the pH of the solution in the reaction process is in the alkali range, granular magnetite particle mix with lepidocrocite particles.
If the pH of the solution in the reaction process is as low as less than 7, when the precipitate containing an alkali metal and an SO.sub.4 which are produced at the same time with the production of lepidocrocite particles, that is, a slightly soluble sulfur-containing iron salt represented by RFe.sub.3 (SO.sub.4).sub.2 (OH).sub.6 (where R represents K.sup.+, Na.sup.+ or NH.sup.+) is contained in particles and/or is present between particles. Since the slightly soluble sulfur-containing iron salt is difficult to remove and remains crosslinked between particles when it is washed with water, so that agglomerates are likely to be caused.
When lepidocrocite particles are produced at a temperature of not more than 15.degree. C., cooling or the like is necessitated, which is industrially disadvantageous.
In case of the above-described process (4), the reaction at as high a temperature as 55 to 100.degree. C. at the time of growing seed crystals can be conducted if a water-soluble phosphorus compound or arsenic compound is present at the time of producing the seed crystals. The form of the seed crystals is transformed from a non-isometric system to an isometric system so as to obtain lepidocrocite particles having an isometric system, i.e. granular lepidocrocite particles.
In the lepidocrocite particles produced by these methods, however, since the lepidocrocite particles are granular, the tinting strength and the hiding power are insufficient.
In the production of seed crystals, the lepidocrocite particles have an acicular shape and granular magnetite particles mix with the lepidocrocite particles, as will be shown in later-described comparative examples.
Accordingly, the present invention is aimed at industrially and economically producing lepidocrocite particles which are excellent in dispersibility, tinting strength and hiding power.
The present invention is also aimed at producing non-magnetic particles used for a non-magnetic undercoat layer formed on a non-magnetic substrate so as to obtain a substrate having an excellent surface smoothness and a high strength.
As a result of studies undertaken by the present inventors, it has been found that by mixing an aqueous ferrous sulfate, an aqueous alkali hydroxide and 0.1 to 5.0 mol % of at least one selected from the group consisting of a phosphorus compound and a citric compound based on Fe at a temperature of 25 to 55.degree. C. to produce a suspension containing an iron hydroxide and having a pH of 7 to 9, and oxidizing the iron hydroxide by passing an oxygen-containing gas into the suspension while adjusting the pH value to 7 to 9 at a temperature of 25 to 55.degree. C., the obtained rectangular parallelopipedic lepidocrocite particles having a minor axial diameter of 0.045 to 0.5 .mu.m, a major axial diameter of 0.05 to 1.0 .mu.m, a thickness of 0.001 to 0.3 .mu.m and a geometrical standard deviation of said major axial diameter of preferably not more than 1.70, more preferably not more than 1.40, are individual and are excellent in dispersibility, tinting strength and hiding power. The present invention has been achieved on the basis of this finding.